Device and method for fluid aeration via gas forced through a liquid within an orifice of a pressure chamber

ABSTRACT

The present invention provides aeration methods using spherical gas bubbles having a size on the order of 0.1 to 100 microns in size. A device of the invention for producing a monodispersion of bubbles includes a source of a stream of gas which is forced through a liquid held under pressure in a pressure chamber with an exit opening therein. The stream of gas surrounded by the liquid in the pressure chamber flows out of an exit orifice of the chamber into a liquid thereby creating a monodispersion of bubbles with substantially uniform diameter. The bubbles are small in size and produced with a relatively small amount of energy relative to comparable systems. Applications of the aeration technology range from oxygenating sewage with monodispersions of bubbles to oxygenation of water for fish maintenance.

This application is a continuation of U.S. application Ser. No.09/932,225, filed Aug. 17, 2001, now U.S. Pat. No. 6,394,429, which is acontinuation of U.S. application Ser. No. 09/604,834, filed Jun. 27,2000, now U.S. Pat. No. 6,299,145 issued Oct. 9, 2001 which is acontinuation of application Ser. No. 09/191,756 filed Nov. 13, 1998 nowU.S. Pat. No. 6,196,525 issued Mar. 6, 2001 which application is acontinuation-in-part of U.S. application Ser. No. 09/171,518, filed Oct.20, 1998 now U.S. Pat. No. 6,119,953 issued Sep. 19, 2000 whichapplication is a continuation-in-part of U.S. application Ser. No.09/192,091, filed Nov. 13, 1998 now U.S. Pat. No. 6,116,516 issued Sep.12, 2000 which application is the National Phase of PCT ApplicationSerial No. PCT/ES97/00034, filed Feb. 18, 1997; which application claimspriority to Spanish Applications P9601101, filed May 13, 1996 andP9702654, filed Dec. 17, 1997.

FIELD OF THE INVENTION

The invention relates generally to the field of small particle formationand more specifically to fields where it is important to create gasbubbles which are very small and uniform in size.

BACKGROUND OF THE INVENTION

Monodispersed sprays of droplets of micrometric size have attracted theinterest of scientist and engineers because of their potentialapplications in many fields of science and technology. Classifying apolydispersed aerosol (for example, by using a differential mobilityanalyzer, B. Y. Liu et al. (1974), “A Submicron Standard and the PrimaryAbsolute Calibration of the Condensation Nuclei Counter,” J. ColoidInterface Sci. 47:155-171 or breakup process of Rayleigh's type of acapillary microjet Lord Rayleigh (1879),“On the instability of Jets,”Proc. London Math. Soc. 10:4-13, are the current methods to produce themonodispersed aerosols of micrometric droplets needed for suchapplications. The substantial loss of the aerosol sample during theclassification process can severely limit the use of this technique forsome applications. On the other hand, although in the capillary break upthe size distribution of the droplets can be very narrow, the diameterof the droplets is determined by the jet diameter (approximately twicethe jet diameter). Therefore, the generation and control of capillarymicrojets are essential to the production of sprays of micrometricdroplets with very narrow size distribution.

Capillary microjets with diameters ranging from tens of nanometers tohundred of micrometers are successfully generated by employing highelectrical fields (several kV) to form the well-known cone-jetelectrospray. Theoretical and experimental results and numericalcalculations on electrosprays can be obtained from M. Cloupean et al.(1989),“Electrostatic Spraying of Liquids in Cone Jet Mode,” J.Electrostat 22:135-159, Fernandez de la Mora et al. (1994),“The CurrentTransmitted through an Electrified Conical Meniscus,” J. FluidMech.260:155-184 and Loscertales (1994), A. M. G{umlaut over(n)}án-Calvo et al. (1997),“Current and Droplet Size in theElectrospraying of Liquids: Scaling Laws,” J. Aerosol Sci. 28:249-275,Hartman et al. (1997).“Electrohydrodynamic Atomization in the Cone-JetMode.” Paper presented at the ESF Workshop on Electrospray, Sevilla,Feb. 28-Mar. 1, 1997 among others [see also the papers contained in theSpecial Issue for Elecrosprays (1994)]. In the electrospray techniquethe fluid to be atomized is slowly injected through a capillaryelectrified needle. For a certain range of values of the applied voltageand flow rate an almost conical meniscus is formed at the needle's exitfrom whose vertex a very thin, charged jet is issued. The jet breaks upinto a fine aerosol of high charged droplets characterized by a verynarrow droplet size distribution. Alternatively, the use of purelymechanical means to produce capillary microjets is limited in most ofapplications for several reasons: the high-pressure values required toinject a fluid through a very narrow tube (typical diameters of theorder of few micrometers) and the easy clogging of such narrow tubes dueto impurities in the liquid.

The present invention provides a new technique for producing uniformsized monodispersion of gas bubbles based on a mechanical means whichdoes not present the above inconveniences and can compete advantageouslywith electrospray atomizers. The jet diameters produced with thistechnique can be easily controlled and range from below one micrometerto several tens of micrometers.

SUMMARY OF THE INVENTION

The present invention provides aeration methods using spherical gasbubbles having a size on the order of 0.1 to 100 microns in size. Adevice of the invention for producing a monodispersion of bubblesincludes a source of a stream of gas which is forced through a liquidheld under pressure in a pressure chamber with an exit opening therein.The stream of gas surrounded by the liquid in the pressure chamber flowsout of an exit orifice of the chamber into a liquid thereby creating amonodispersion of bubbles with substantially uniform diameter. Thebubbles are small in size and produced with a relatively small amount ofenergy relative to comparable systems. Applications of the aerationtechnology range from oxygenating sewage with monodispersions of bubblesto oxygenation of water for fish maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the basic components of oneembodiment of the invention with a cylindrical feeding needle as asource of formulation.

FIG. 2 is a schematic view of another embodiment of the invention withtwo concentric tubes as a source of formulation.

FIG. 3 is a schematic view of yet another embodiment showing awedge-shaped planar source of formulation. FIG. 3a illustrates across-sectional side view of the planar feeding source and theinteraction of the fluids. FIG. 3b show a frontal view of the openingsin the pressure chamber, with the multiple openings through which theatomizate exits the device. FIG. 3c illustrates the channels that areoptionally formed within the planar feeding member. The channels arealigned with the openings in the pressure chamber.

FIG. 4 is a schematic view of a stable capillary microjet being formedand flowing through an exit opening to thereafter form a monodisperseaerosol.

FIG. 5 is a graph of data where 350 measured values of d_(j)/d_(o)versus Q/Q_(o) are plotted.

FIG. 6 is a micrograph showing the even dispersement and uniform size ofair bubbles created using the method of the invention after expulsioninto an aqueous solution.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Before the present aeration device and method are described, it is to beunderstood that this invention is not limited to the particularcomponents and steps described, as such may, of course, vary. It is alsoto be understood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting, since the scope of the present invention will be limited onlyby the appended claims.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “and,” and “the” include plural referents unless thecontext clearly dictates otherwise. Thus, for example, reference to “abubble” includes a plurality of bubbles and reference to “a gas”includes reference to a mixture of gases, and equivalents thereof knownto those skilled in the art, and so forth.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, the preferred methodsand materials are now described. All publications mentioned herein areincorporated herein by reference to disclose and describe the methodsand/or materials in connection with which the publications are cited.

The publications discussed herein are provided solely for theirdisclosure prior to the filing date of the present application. Nothingherein is to be construed as an admission that the present invention isnot entitled to antedate such publication by virtue of prior invention.Further, the dates of publication provided may be different from theactual publication dates which may need to be independently confirmed.

Definitions

The terms “bubble”, “dispersion of bubbles” and “monodispersion ofbubbles” are used interchangeably herein and shall mean small uniformlysized particles of a gas or gaseous formulation that has been dispersedusing the device and method of the invention. The particles aregenerally spherical, and may be comprised of one or more gases or layersof gases.

The terms “air”, “particle free air” and the like, are usedinterchangeably herein to describe a volume of air which issubstantially free of other material and, in particular, free ofparticles intentionally added such as particles of formulation. Air is amixture of various gas components that may, of course vary, but usuallythe air will contain approximately 21% oxygen by volume. Air may alsocontain gases or other air-borne particles. For use in the invention,air may be filtered or treated to remove all unwanted particulate orgaseous matter, or the air may be used in an unfiltered state. Air isthe preferred gas for use of the invention in oxygenation of aqueousfluids, e.g. water.

The terms “gas” and “gas formulation” as used herein refer to any gas orgaseous mixture which is desired to be dispersed using the method of theinvention. For example, the formulation may be comprised of air, eitherfiltered or unfiltered. Gases such as air may be spiked with aparticular gas, such as the spiking of air with additional O₂ gas foruse in oxygenation. A gaseous formulation may also contain suspendedparticulate matter dispersed within the gas. The gas can be CO2 to carryout the carbonation of beverages (e.g. water, colas) or a gas containingan unwanted contaminant, e.g. radioactivity or an environmental

The term“aeration” as used herein refers to the dispersion of a, gaseousmaterial into a flowable fluid, for example to provide a diffusionsurface to introduce a molecule or come pound from the gas into theflowable surface. The term is not limited to the dispersion of air perse, although the use of air is preferred, but rather refers to theintroduction of any gas to a flowable fluid, e.g. O₂, CO_(2,) hydrogen,nitrogen, and the like and mixtures thereof. The aeration of a fluid ispreferably to allow molecules and/or compounds to diffuse to the fluidthrough the fluid-bubble interface following expulsion of the bubblesfrom the device of the invention into the surrounding fluid. A fluidmay, however, also be aerated for aesthetic purposes, such as theaddition of CO2 to a beverage to provide carbonation.

Device in General

Different embodiments are shown and described herein (see FIGS. 1, 2 and3) which could be used in producing the stable capillary microjet and/ora dispersion of particles which are substantially uniform in size.Although various embodiments are part of the invention, they are merelyprovided as exemplary devices which can be used to convey the essence ofthe invention, which is the formation of a stable capillary microjetand/or uniform dispersion of particles.

A basic device comprises (1) a means for supplying a first fluid,preferably a gas, and (2) a pressure chamber supplied with a secondfluid which flows out of an exit opening in the pressure chamber,preferably a liquid. The exit opening of the pressure chamber is alignedwith the flow path of the means for supplying the first fluid. Theembodiments of FIGS. 1, 2 and 3 clearly show that there can be a varietyof different means for supplying the first fluid. Other means forsupplying a first fluid flow stream will occur to those skilled in theart upon reading this disclosure.

Further, other configurations for forming the pressure chamber aroundthe means for supplying the first fluid will occur to those skilled inthe art upon reading this disclosure. Such other embodiments areintended to be encompassed by the present invention provided the basicconceptual results disclosed here are obtained, i.e. a stable capillarymicrojet is formed and/or a dispersion of particle highly uniform insize is formed. To simplify the description of the invention, the meansfor supplying a first fluid is often referred to as a cylindrical tube(see FIG. 1) and the first fluid is generally referred to as a gas. Thegas can be any as depending on the desired use of the device, althoughit is preferably air. For example, the gas could be air used to createsmall bubbles for aeration of a liquid to provide a gaseous medium,through which components may diffuse into a liquid. Further, forpurposes of simplicity, the second fluid is generally described hereinas being a liquid, e.g. water. The invention is also generally describedwith a gas formulation being expelled from the supply means and forminga stable microjet due to interaction with surrounding water flow, whichfocuses the gas microjet to flow out of an exit of the pressure chamber.

Formation of the microjet and its acceleration and ultimate particleformation are based on the abrupt pressure drop associated with thesteep acceleration experienced by the gas on passing through an exitorifice of the pressure chamber which holds the second fluid (i.e. theliquid). On leaving the chamber the flow undergoes a large pressuredifference between the liquid and the gas, which in turn produces ahighly curved zone on the liquid surface near the exit port of thepressure chamber and in the formation of a cuspidal point from which asteady microjet flows, provided the amount of the gas drawn through theexit port of the pressure chamber is replenished. Thus, in the same waythat a glass lens or a lens of the eye focuses light to a given point,the flow of the liquid surrounds and focuses the gas into a stablemicrojet. The focusing effect of the surrounding flow of liquid createsa stream of gas which is substantially smaller in diameter than thediameter of the exit orifice of the pressure chamber. This allows thegas to flow out of the pressure chamber orifice without touching theorifice, providing advantages including the feature that the diameter ofthe stream and the resulting particles are smaller than the diameter ofthe exit orifice of the chamber. This is particularly desirable becauseit is difficult to precisely engineer holes which are very small indiameter. Further, in the absence of the focusing effect (and formationof a stable interface cusp) flow of gas out of an opening will result inparticles which have a diameter greater than the diameter of the exitopening.

The description provided here generally indicates that the gas leavesthe pressure chamber through an exit orifice surrounded by the liquidand thereafter enters into a liquid surrounding environment which may beeither a hydrophobic or hydrophilic liquid. This configuration isparticularly useful when it is necessary to create very small highlyuniform bubbles which are moved into a liquid surrounding exit openingof the pressure chamber. The then need for the formation of very smallhighly uniform bubbles into a gas occurs in a variety of differentindustrial applications. For example, water needs to be oxygenated in avariety of situations including small fish tanks for home use and largevolume fisheries or industrial use. The additional oxygen can aid therate of growth of the fish and thereby improve production for thefishery. In another embodiment, oxygen or air bubbles can be forced intoliquid sewage in order to aid in treatment. In yet another applicationof the invention, contaminated gases such as a gas contaminated with aradioactive material can be formed into small uniformed bubbles andblown into a liquid, where the contamination in the gas will diffuseinto the liquid, thereby cleaning the gas. The liquid will, of course,occupy substantially less volume and therefore be substantially easierto dispose of than contaminated toxic gas.

Those skilled in the art will recognize that variations on the differentembodiments disclosed below will be useful in obtaining particularlypreferred results. Specific embodiments of devices are now described.

Embodiment of FIG. 1

A first embodiment of the invention where the supply means is acylindrical feeding needle supplying gas into a pressurized chamber ofliquid is described below with reference to FIG. 1.

The components of the embodiment of FIG. 1 are as follows:

1. Feeding needle—also referred to generally as a fluid source and atube.

2. End of the feeding needle used to insert the gas to be dispersed.

3. Pressure chamber.

4. Orifice used as liquid inlet.

5. End of the feeding needle used to evacuate the liquid to be atomized.

6. Orifice through which withdrawal takes place.

7. Atomizate (spray)—also referred to as aerosol.

D₀=diameter of the feeding needle; d₀=diameter of the orifice throughwhich the microjet is passed; e=axial length of the orifice throughwhich withdrawal takes place; H=distance from the feeding needle to themicrojet outlet: P₀=pressure inside the chamber; P_(a)=atmosphericpressure.

Although the device can be configured in a variety of designs, thedifferent designs will all include the essential components shown inFIG. 1 component which perform an equivalent function and obtain thedesired results. Specifically, a device of the invention will becomprised of at least one source of a first fluid (e.g., a feedingneedle with an opening 2) into which a first fluid such as a gasformulation can be fed and an exit opening 5 from which the gas can beexpelled. The feeding needle, or at least its exit opening 5, isencompassed by a pressure chamber 3. The chamber 3 has inlet opening 4which is used to feed a second fluid (e.g. a liquid) into the chamber 3and an exit opening 6 through which liquid from the pressure chamber andgas from the feeding needle 3 are expelled. When the first fluid is agas it is preferably expelled into a liquid to create bubbles.

In FIG. 1, the feeding needle and pressure chamber are configured toobtain a desired result of producing bubbles wherein the particles aresmall and uniform in size. The bubbles have a size which is in a rangeof 0.1 to 100 microns. The particles of any given bubbles will all haveabout the same diameter with a relative standard deviation of ±10% to±30% or more preferably ±3% to ±10%. Stating that bubbles will have adiameter in a range of 1 to 5 microns does not mean that differentbubbles will have different diameters and that some will have a diameterof 1 micron while others of 5 microns. The bubbles in a given dispersionwill all (preferably about 90% or more) have the same diameter ±3 % to±30%. For example, the bubbles of a given dispersion will have adiameter of 2 microns ±3% to ±10%.

Such a uniform bubble monodispersion is created using the components andconfiguration as described above. However, other components andconfigurations will occur to those skilled in the art. The object ofeach design will be to supply fluid so that it creates a stablecapillary microjet which is accelerated and stabilized by tangentialviscous stress exerted by the second fluid on the first fluid surface.The stable microjet created by the second fluid leaves the pressurizedarea (e.g., leaves the pressure chamber and exits the pressure chamberorifice) and splits into particles or bubbles which have the desiredsize and uniformity.

The parameter window used (i.e. the set of special values for theproperties of the liquid used. flow-rate used feeding needle diameter,orifice diameter, pressure ratio, etc.) should be large enough to becompatible with virtually any liquid (dynamic viscosities in the rangefrom 10⁻⁴ to 1 kg m⁻¹ S⁻¹); in this way the capillary microjet thatemerges from the end of the feeding needle is absolutely stable andperturbations produced by breakage of the jet cannot travel upstream.Downstream, the microjet splits into evenly shaped bubbles simply byeffect of capillary instability (see, for example, Rayleigh,“On theinstability of jets”, Proc. London Math. Soc., 4-13, 1878), similar in amanner to a laminar capillary jet falling from a half-open tap.

When the stationary, steady interface is created, the capillary jet thatemerges from the end of the drop at the outlet of the feeding point isconcentrically withdrawn into the nozzle. After the jet emerges from thedrop, the liquid is accelerated by tangential sweeping forces exerted bythe gas stream flowing on its surface, which gradually decreases the jetcross-section. Stated differently the liquid flow acts as a lens andfocuses and stabilizes the microjet as it moves toward and into the exitorifice of the pressure chamber. When the first fluid of the inventionis a gas, and the second fluid is a liquid, the inertia of the firstfluid is low, and the gas abruptly decelerates very soon after it issuesfrom the cusp of the attached droplet. In such an instance, the microjetis so short that it is almost indistinguishable from the stable cusp ofthe gas-liquid interface.

The forces exerted by the second fluid flow on the first fluid surfaceshould be steady enough to prevent irregular surface oscillations.Therefore, any turbulence in the gas motion should be avoided; even ifthe gas velocity is high, the characteristic size of the orifice shouldensure that the gas motion is laminar (similar to the boundary layersformed on the jet and on the inner surface of the nozzle or hole).

Stable Capillary Microjet

FIG. 4 illustrates the interaction of a gas and a liquid to form bubblesusing the method of the invention. The feeding needle 60 has a circularexit opening 61 with an internal radius R_(o) which feeds a gas 62 outof the end, forming a drop with a radius in the range of R_(o) to R_(o)plus the thickness of the wall of the needle. The exiting (as forms aninfinite amount of streamlines 63 that interact with the surroundingliquid to form a stable cusp at the interface 64 of the two fluids. Thesurrounding liquid also forms an infinite number of liquid streamlines65. which interact with the exiting gas to create a virtual focusingfunnel 66. The exiting gas is focused by the focusing funnel 66resulting in a stable capillary/microjet 67, which remains stable untilit exits the opening 68 of the pressure chamber 69. After exiting thepressure chamber, the microjet begins to break-up, forming monodispersedparticles 70.

The liquid flow, which affects the gas withdrawal and its subsequentdeceleration after the jet is formed, should be very rapid but alsouniform in order to avoid perturbing the fragile capillary interface(the surface of the drop that emerges from the jet).

Gas flows out of the end of a capillary tube and forms a small gas dropat the end. The tube has an internal radius R_(o). The drop has a radiusin a range of from R_(o) to R_(o) plus the structural thickness of thetube as the drop exits the tube, and thereafter the drop narrows incircumference to a much smaller circumference as is shown in theexpanded view of the tube (i.e. feeding needle) 5 as shown in FIGS. 1and 4.

As illustrated in FIG. 4, the exit opening 61 of the capillary tube 60is positioned close to an exit opening 68 in a planar surface of apressure chamber 69. The exit opening 68 has a minimum diameter D and isin a planar member with a thickness L. The diameter D is referred to asa minimum diameter because the opening may have a conical configurationwith the narrower end of the cone positioned closer to the source ofliquid flow. Thus, the exit opening may be a funnel-shaped nozzlealthough other opening configurations are also possible, e.g. an hourglass configuration. Liquid in the pressure chamber continuously flowsout of the exit opening. The flow of the liquid causes the gas dropexpelled from the tube to decrease in circumference as the gas movesaway from the end of the tube in a direction toward the exit opening ofthe pressure chamber.

In actual use, it can be understood that the opening shape whichprovokes maximum liquid acceleration (and consequently the most stablecusp and microjet with a given set of parameters) is a conically shapedopening in the pressure chamber. The conical opening is positioned withits narrower end toward the source of gas flow.

The distance between the end 61 of the tube 60 and the beginning of theexit opening 68 is H. At this point it is noted that R_(o), D, H and Lare all preferably on the order of hundreds of microns. For example,R_(o)=400 μm, D=150 μm, H=1 mm, L=300 μm. However, each could be{fraction (1/100)} to 100×these sizes. The end of the gas streamdevelops a cusp-like shape at a critical distance from the exit opening68 in the pressure chamber 69 when the applied pressure drop ΔP_(g)across the exit opening 68 overcomes the liquid-gas surface tensionstresses γ/R appearing at the point of maximum curvature—e.g. 1/R fromthe exit opening.

A steady state is then established if the gas flow rate Q elected fromthe drop cusp is steadily supplied from the capillary tube. This is thestable capillary cusp which is an essential characteristic of theinvention needed to form the stable microjet. More particularly, asteady, thin gas jet with a typical diameter d_(j) is smoothly emittedfrom the stable cusp-like drop shape and this thin gaseous jet extendsover a distance in the range of microns to millimeters. The length ofthe stable microjet will vary from very short (e.g. 1 micron) to verylong (e.g. 50 mm) with the length depending on the (1) flow-rate of thegas and (2) the Reynolds number of the gas stream flowing out of theexit opening of the pressure chamber. The gas jet is the stablecapillary microjet obtained when supercritical flow is reached. Asmentioned, in the case of a gas jet the microjet may be so small as tobe almost indistinguishable from the stable cusp. This jet demonstratesa robust behavior provided that the pressure drop ΔP_(l) applied to theliquid is sufficiently large compared to the maximum surface tensionstress (on the order of γ/d_(j)) that act at the liquid-gas interface.The jet has a slightly parabolic axial velocity profile which is, inlarge part, responsible for the stability of the microjet. The stablemicrojet is formed without the need for other forces, i.e. withoutadding force such as electrical forces on a charged fluid. However, forsome applications it is preferable to add charge to particles, e.g. tocause the particles to adhere to a given surface. The shaping of liquidexiting the capillary tube by the gas flow forming a focusing funnelcreates a cusp-like meniscus resulting in the stable microjet. This is afundamental characteristic of the invention.

The microjet eventually destabilizes due to the effect of surfacetension forces. Destabilization results from small natural perturbationsmoving downstream, with the fastest growing perturbations being thosewhich govern the break up of the microjet, eventually creating a uniformsized monodispersion of bubbles 70 as shown in FIG. 4. The microjet,even as it initially destabilizes, passes out of the exit orifice of thepressure chamber without touching the peripheral surface of the exitopening.

Mathematics of a Stable Microjet

Cylindrical coordinates (r,z) are chosen for making a mathematicalanalysis of a stable microjet, i.e. fluid undergoing“supercriticalflow.” The cusp-like meniscus formed by the fluid coming out of the tubeis culled toward the exit of the pressure chamber by a pressure gradientcreated by the flow of a second, immiscible fluid.

The cusp-like meniscus formed at the tube's mouth is pulled towards thehole by the pressure gradient created by the liquid stream. From thecusp of this meniscus, a steady gas thread with the shape of radius r=ξis withdrawn through the hole by the action of both the suction effectdue to ΔP_(l), and the tangential viscous stresses τ_(s) exerted by theliquid on the jet's surface in the axial direction. The averagedmomentum equation for this configuration may be written: $\begin{matrix}{{{\frac{d}{d_{z}}\left\lbrack {P_{g} + \frac{\rho_{g}Q^{2}}{2\pi^{2}\xi^{4}}} \right\rbrack} = \frac{2\tau_{s}}{\xi}},} & (1)\end{matrix}$

where Q is the gas flow rate upon exiting the feeding tube, P_(g) is thegas pressure, and ρ_(g) is the gas density, assuming that the viscousextensional term is negligible compared to the kinetic energy term, aswill be subsequently justified. The gas pressure P_(g) is given by thecapillary equation.

P _(g) =P _(l)+γ/ξ.  (2)

where γ is the liquid-gas surface tension. As shown in the Examples, thepressure drop ΔP_(l) is sufficiently large as compared to the surfacetension stress γ/ξ to justify neglecting the latter in the analysis.This scenario holds for the whole range of flow rates in which themicrojet is absolutely stable. In fact, it will be shown that, for agiven pressure drop ΔP_(l), the minimum liquid flow rate that can besprayed in steady jet conditions is achieved when the surface tensionstress Δ/ξ is of the order of the kinetic energy of the liquidρ₁Q²/(2π²ξ⁴), since the surface tension acts like a“resistance” to themotion (it appears as a negative term in the right-hand side term of Eq.(1)). Thus, $\begin{matrix}{\left. Q_{\min} \right.\sim\left( \frac{\gamma \quad d_{j}^{3}}{\rho_{g}} \right)^{\frac{1}{2}}} & (3)\end{matrix}$

For sufficiently large flow rates Q compared to Q_(min), the simplifiedaveraged momentum equation in the axial direction can be expressed as$\begin{matrix}{{{\frac{d}{d_{z}}\left( \frac{\rho_{g}Q^{2}}{2\pi^{2}\xi^{4}} \right)} = {\frac{{dP}_{l}}{d_{z}} + \frac{2\tau_{s}}{\xi}}},} & (4)\end{matrix}$

where one can identify the two driving forces for the gas flow on theright-hand side. This equation can be integrated provided the followingsimplification is made: if one uses a thin plate with thickness L of theorder or smaller than the hole's diameter D (which minimizes downstreamperturbations in the liquid flow), the pressure gradient up to the holeexit is on the average much larger than the viscous shear term 2τ_(s)/ξowning to the surface stress. On the other hand, the axial viscous termis of the order O[μ²Q/D²d_(j) ²], since the hole diameter D is actuallythe characteristic distance associated with the gas flow at the hole'sentrance in both the radial and axial directions. This term is verysmall compared to the pressure gradient in real situations, providedthat ΔP_(l)>) μ²/D²ρ_(g) (which holds, e.g., for liquids withviscosities as large as 100 cpoises, using hole diameters and pressuredrops as small as D˜10 μm and ΔP_(g)≧100 mbar). The neglect of allviscous terms in Eq. (4) is then justified. Notice that in this limit onthe liquid flow is quasi-isentropic in the average (the liquid almostfollows Bernoulli equation) as opposed to most micrometric extensionalflows. Thus, integrating (4) from the stagnation regions of both fluidsup to the exit, one obtains a simple and universal expression for thejet diameter at the hole exit: $\begin{matrix}{{d_{j} \simeq {\left( \frac{8\rho_{g}}{\pi^{2}\Delta \quad P_{l}} \right)^{\frac{1}{4}}Q^{\frac{1}{2}}}},} & (5)\end{matrix}$

which for a given pressure drop ΔP_(l) is independent of geometricalparameters (hole and tube diameters, tube-hole distance. etc.), liquidand gas viscosities, and liquid-gas surface tension. This diameterremains almost constant up to the breakup point since the as pressureafter the exit remains constant.

Monodisperse Particles

Above the stable microjet undergoing“supercritical flow” is describedand it can be seen how this aspect of the invention can be made use ofin a variety of industrial applications. When the microjet exits thepressure chamber the gas pressure P_(g) becomes (like the liquidpressure P_(l)) almost constant in the axial direction, and the jetdiameter remains almost constant up to the point where it breaks up bycapillary instability. Defining a Weber number We=(ρ_(l)v_(l)²d_(j))/γ≅2 ΔP_(l)d_(j)/γ(where v_(l) is the liquid velocity measured atthe orifice), below a certain experimental value We_(c)˜40 the breakupmode is axisymmetric and the resulting droplet stream is characterizedby its monodispersity provided that the fluctuations of the gas flow donot contribute to droplet coalescence (these fluctuations occur when thegas stream reaches a fully developed turbulent profile around the liquidjet breakup region). Above this We_(c) value, sinuous nonaxisymmetricdisturbances, coupled to the axisymmetric ones, become apparent. Forlarger We numbers, the nonlinear growth rate of the sinuous disturbancesseems to overcome that of the axisymmetric disturbances. The resultingspray shows significant polydispersity in this case. Thus, it can beseen that by controlling parameters to keep the resulting Weber numberto 40 or less, allows the bubbles formed to be all substantially thesame size. The size variation is about ±3% to ±30% and move preferably±3% to ±10%. These particles can have a desired size e.g. 0.1 microns to50 microns.

The shed vorticity influences the breakup of the jet and thus theformation of the particles. Upstream from the hole exit, in theaccelerating region, the gas stream is laminar. Typical values of theReynolds number range from 500 to 6000 if a velocity of the order of thespeed of sound is taken as characteristic of the velocity of the liquid.Downstream from the hole exit, the cylindrical mixing layer between thegas stream and the stagnant gas becomes unstable by the classicalKelvin-Helmholtz instability. The growth rate of the thickness of thislayer depends on the Reynolds number of the flow and ring vortices areformed at a frequency of the order of v_(l)/D, where D is the holediameter. Typical values of v_(l) and D as those found in ourexperimental technique lead to frequencies or the order of MHZ which arecomparable to the frequency of drop production (of order of I_(b) ⁻¹).

Given the gas flow rate and the hole diameter, a resonance frequencywhich depends on the gas velocity (or pressure difference driving thegas stream) can be adjusted (tuned) in such a way that vortices act as aforcing system to excite perturbations of a determined wavelength on thejet surface. Experimental results obtained clearly illustrates thedifferent degree of coupling between the two gas-liquid coaxial jets. Inone set of experimental results the bubble sizes are shown to have abubble size of about 5.7 microns with a standard deviation of 12%. Thisresults when the velocity of the liquid has been properly tuned tominimize the dispersion in the size of droplets resulting from the jetbreakup. In this case, the flow rate of the gas jet and its diameter are0.08 μl s⁻¹ and 3 μm, respectively. Data have been collected using aMASTERSIZER from MALVERN Instruments. As the degree of couplingdecreases, perturbations at the jet surface of different wavelengthsbecome excited and, as it can be observed from the size distributions,the dispersion of the spray increases.

The liquid flow should be laminar in order to avoid a turbulentregime—turbulent fluctuations in the gas flow which have a highfrequency and would perturb the liquid-gas interface. The Reynoldsnumbers reached at the orifice are${Re} = {\left. \frac{v_{l}d_{0}}{v_{l}} \right.\sim 4000}$

where v_(l) is the kinematic viscosity of the liquid. Even though thisnumber is quite high, there are large pressure gradients downstream (ahighly convergent geometry), so that a turbulent regime is very unlikelyto develop.

The essential difference from existing pneumatic atomizers (whichpossess large Weber numbers) and the present invention is that the aimof the present invention is not to rupture the liquid-gas interface butthe opposite, i.e. to increase the stability of the interface until acapillary jet is obtained. The jet, which will be very thin provided thepressure drop resulting from withdrawal is high enough, splits intodrops the sizes of which are much more uniform than those resulting fromdisorderly breakage of the liquid(l,,s interface In existing pneumaticatomizers.

The proposed system obviously requires delivery of the gas to beatomized and the liquid to be used in the resulting drop production.Both should be fed at a rate ensuring that the system lies within thestable parameter window. Multiplexing is effective when the flow-ratesneeded exceed those on an individual cell. More specifically, aplurality of feeding sources or feeding needles may be used to increasethe rate at which aerosols are created. The flow-rates used should alsoensure the mass ratio between the flows is compatible with thespecifications of each application.

The gas and liquid can be dispensed by any type of continuous deliverysystem (e.g. a compressor or a pressurized tank the former and avolumetric pump or a pressurized bottle the latter). If multiplexing isneeded, the liquid flow-rate should be as uniform as possible amongcells; this may entail propulsion through several capillary needles,porous media or any other medium capable of distributing a uniform flowamong different feeding points.

Each individual device should consist of a feeding point (a capillaryneedle, a point with an open microchannel, a microprotuberance on acontinuous edge, etc.) 0.002-2 mm (but, preferentially 0.01-0.4 mm) indiameter, where the drop emerging from the microjet can be anchored, anda small orifice 0.002-2 mm (preferentially 0.01-0.25 mm) in diameterfacing the drop and separated 0.01-2 mm (preferentially 0.2-0.5 mm) fromthe feeding point. The orifice communicates the withdrawal liquid aroundthe drop, at an increased pressure, with the zone where the atomizate isproduced, at a decreased pressure. The device can be made from a varietyof materials (metal, polymers, ceramics, glass).

FIG. 1 depicts a tested prototype where the gas to be atomized isinserted through one end of the system 2 and the liquid in introducedvia the special inlet 4 in the pressure chamber 3. The prototype wastested at gas feeding rates from 100 to 2000 mBar above the atmosphericpressure P_(a) at which the atomized gas was discharged. The wholeenclosure around the feeding needle 1 was at a pressure P_(o)>P_(a). Thegas feeding pressure, P_(l), should always be slightly higher than thegas propelling pressure, P_(o). Depending on the pressure drop in theneedle and the gas feeding system, the pressure difference(P_(l)−P_(o)>0) and the flow-rate of the gas to be atomized, Q arelinearly related provided the flow is laminar—which is indeed the casewith this prototype. The critical dimensions are the distance from theneedle to the plate (H), the needle diameter (D_(O)) the diameter of theorifice through which the microjet 6 is discharged (d₀) and the axiallength, e, of the orifice (i.e. the thickness of the plate where theorifice is made). In this prototype. H was varied from 0.3 to 0.7 mm onconstancy of the distances (D_(O)=0.45 mm, d_(O)−0.2 mm) and e −0.5 mm.The quality of the resulting spray 7 did not vary appreciably withchanges in M provided the operating regime (i.e. stationary drop andmicrojet) was maintained. However, the system stability suffered at thelonger H distances (about 0.7 mm). The other atomizer dimensions had noeffect en the spray or the prototype functioning provided the zonearound the needle (its diameter) was large enough relative to thefeeding needle.

Weber Number

Adjusting parameters to obtain a stable capillary microjet and controlits breakup into monodisperse particle is governed by the Weber numberand the liquid-to-gas velocity ratio or a which equal V_(l)/V_(g). TheWeber number or “We” is defined by the following equation:${We} = \frac{\rho_{l}V_{l}^{2}d}{\gamma}$

wherein ρ_(l) is the density of the gas, d is the diameter of the stablemicrojet, γ is the liquid-gas surface tension, and V_(l) ² is thevelocity of the gas squared.

When carrying out the invention the parameters should be adjusted sothat the Weber number is greater than 1 in order to produce a stablecapillary microjet. However, to obtain a particle dispersion which ismonodisperse (i.e. each particle has the same size ±3 to ±30%) theparameters should be adjusted so that the Weber number is less than 40.The monodisperse aerosol is obtained with a Weber number in a range ofabout 1 to about 40 when the breaking time is sufficiently small toavoid non-symmetric perturbations. (1≦We≦40)

Ohnesorge Number

A measure of the relative importance of viscosity on the jet breakup canbe estimated from the Ohnesorge number defined as the ratio between twocharacteristic times: the viscous time t_(v) and the breaking timet_(b). The breaking time t_(b) is given by [see Rayleigh (1878)]$\begin{matrix}{{\left. t_{b} \right.\sim\left( \frac{\rho_{g}d^{2}}{\gamma} \right)^{\frac{1}{2}}}.} & (2)\end{matrix}$

Perturbations on the jet surface are propagated inside by viscousdiffusion in times t_(v) of the order of

t_(v)−ρ_(g)d²/μ_(g),  (3)

where μ₁ is the viscosity of the liquid. Then, the Ohnesorge number, Oh,results $\begin{matrix}{{Oh} = {\frac{\mu_{g}}{\left( {\rho_{g}\gamma \quad d} \right)^{\frac{1}{2}}}.}} & (4)\end{matrix}$

If this ratio is much smaller than unity viscosity plays no essentialrole in the phenomenon under consideration. Since the maximum value ofthe Ohnesorge number in actual experiments conducted is as low as3.7×10^(−2,) viscosity plays no essential role during the process of jetbreakup.

Embodiment of FIG. 2

A variety of configurations of components and types of fluids willbecome apparent to those skilled in the art upon reading thisdisclosure. These configurations and fluids are encompassed by thepresent invention provided they can produce a stable capillary microjetof a first fluid from a source to an exit port of a pressure chambercontaining a second fluid. The stable microjet is formed by the firstfluid flowing from the feeding source to the exit port of the pressurechamber being accelerated and stabilized by tangential viscous stressexerted by the second fluid in the pressure chamber on the surface ofthe first fluid forming the microjet. The second fluid forms a focusingfunnel when a variety of parameters are correctly tuned or adjusted. Forexample, the speed, pressure, viscosity and miscibility of the first andsecond fluids are chosen to obtain the desired results of a stablemicrojet of the first fluid focused into the center of a funnel formedwith the second fluid. These results are also obtained by adjusting ortuning physical parameters of the device, including the size of theopening from which the first fluid flows, the size of the opening fromwhich both fluids exit, and the distance between these two openings.

The embodiment of FIG. I can, itself, be arranged in a variety ofconfigurations. Further, as indicated above, the embodiment may includea plurality of feeding needles. A plurality of feeding needles may beconfigured concentrically in a single construct, as shown in FIG. 2.

The components of the embodiment of FIG. 2 are as follows:

21. Feeding needle—tube or source of fluid.

22. End of the feeding needle used to insert the liquids to be atomized.

23. Pressure chamber.

24. Orifice used as liquid inlet.

25. End of the feeding needle used to evacuate the gas to be atomized.

26. Orifice through which withdrawal takes place.

27. Atomizate (spray) or aerosol.

28. First gas to be atomized (inner core of particle).

29. Second fluid to be atomized (outer coating of particle).

30. Liquid for creation of microjet.

31. Internal tube of feeding needle.

32. External tube of feeding needle.

D=diameter of the feeding needle; d=diameter of the orifice throughwhich the microjet is passed; e=axial length of the orifice throughwhich withdrawal takes place; H=distance from the feeding needle to themicrojet outlet; γ=surface tension; P_(O)=pressure inside the chamber;P_(a)=atmospheric pressure.

The embodiment of FIG. 2 is preferably used when attempting to form aspherical particle of one substance surrounded by another substance. Thedevice of FIG. 2 is comprised of the same basic component as per thedevice of FIG. 1 and further includes a second feeding source 32 whichis positioned concentrically around the first cylindrical feeding source31. The second feeding source may be surrounded by one or moreadditional feeding sources with each concentrically positioned aroundthe preceding source.

The process is based on the microstructure which the liquid-gas orliquid-liquid interphase undergoes (if both are immiscible), when saidinterphase approaches a point beginning from which one of the fluids issuctioned off white the combined suction of the two fluids is produced.The interaction causes the fluid physically surrounded by the other toform a capillary microjet which finally breaks into spherical drops. Ifinstead of two fluids (gas-liquid), three or more are used that flow ina concentric manner by injection using concentric tubes, a capillary jetcomposed of two or more layers of different fluids is formed which, whenit breaks, gives rise to the formation of spheres composed of severalapproximately concentric spherical layers of different fluids. The sizeof the outer sphere (its thickness) and the size of the inner sphere(its volume) can be precisely adjusted. This can allow the manufactureof layered bubbles for a variety of end uses.

The method is based on the breaking of a capillary microjet composed ofa nucleus of a gas and surrounded by other liquids and gases which arein a concentric manner injected by a special injection head, in such away that they form a stable capillary microjet and that they do not mixby diffusion during the time between when the microjet is formed andwhen it is broken. When the capillary microjet is broken into sphericaldrops under the proper operating conditions, which will be described indetail below, these drops exhibit a spherical nucleus, the size andeccentricity of which can be controlled.

In the case of spheres containing two materials, the injection head 25consists of two concentric tubes with an external diameter on the orderof one millimeter. Through the internal tube 31 is injected the materialthat will constitute the nucleus of the microsphere, while between theinternal tube 31 and the external tube 32 the coating is injected. Thefluid of the external tube 32 joins with the fluid of tube 31 as thefluids exit the feeding needle, and the fluids thus injected areaccelerated by a stream of gas or liquid that passes through a smallorifice 26 facing the end of the injection tubes. When the drop inpressure across the orifice 26 is sufficient, the fluids form acompletely stationary capillary microjet, if the quantities of liquidsthat are injected are stationary. This microjet does not touch the wallsof the orifice, but passes through it wrapped in the stream of gas orfunnel formed by gas from the tube 32. Because the funnel of fluidfocuses the exiting fluid, the size of the exit orifice 26 does notdictate the size of the particles formed.

When the parameters are correctly adjusted, the movement of the fluid isuniform at the exit of the orifice 26 and the viscosity forces aresufficiently small so as not to alter either the flow or the propertiesof the liquids; for example, if there are biochemical molecularspecimens having a certain complexity and fragility, the viscous forcesthat would appear in association with the flow through a micro-orificemight degrade these substances.

FIG. 2 shows a simplified diagram of the feeding needle 21, which iscomprised of the concentric tubes 31, 32 through the internal andexternal flows of the fluids 28, 29 that are going to compose themicrospheres comprised of two immiscible fluids. The difference inpressures P_(O)−P_(a) (P_(O)>P_(a)) through the orifice 26 establishes aflow of liquid present in the chamber 23 and which is going to surroundthe microjet at its exit. The same pressure gradient that moves theliquid is the one that moves the microjet in an axial direction throughthe hole 26, provided that the difference in pressures P_(O)−P_(a) issufficiently great in comparison with the forces of surface tension,which create an adverse gradient in the direction of the movement.

There are two limitations for the minimum sizes of the inside andoutside jets that are dependent (a) on the surface tensions γ1 of theoutside fluid 29 with the liquid 30 and γ2 of the outside fluid 29 withthe inside fluid (e.g. gas) 28, and (b) on the difference in pressuresΔP=P_(O)−P_(a) through the orifice 26. In the first place, the jump inpressures ΔP must be sufficiently great so that the adverse effects ofthe surface tension are minimized. This, however, is attained for verymodest pressure increases: for example, for a 10 micron jet of a gashaving a surface tension of 0.05 N/m (tap water), the necessary minimumjump in pressure is in the order of 0.05 (N/m) /0.00001 m=ΔP=50 mBar.But, in addition, the breakage of the microjet must be regular andaxilsymmetric, so that the drops will have a uniform size, while theextra pressure ΔP cannot be greater than a certain value that isdependent on the surface tension of the outside gas with the gas γ1 andon the outside diameter of the microjet. It has been experimentallyshown that this difference in pressures cannot be greater than 20 timesthe surface tension γ1 divided by the outside radius of the microjet.

Therefore, given some inside and outside diameters of the microjet,there is a range of operating pressures between a minimum and a maximum;nonetheless, experimentally the best results are obtained for pressuresin the order of two to three times the minimum.

The viscosity values of the gases must be such that the gases with thegreater viscosity μ_(max) verifies, for a diameter d of the jetpredicted for this gas and a difference through the orifice ΔP, theinequality: $\mu_{\max} \leq \frac{\Delta \quad {Pd}^{2}D}{Q}$

With this, the pressure gradients can overcome the extensional forces ofviscous resistance exerted by the gas when it is suctioned toward theorifice.

Moreover, the gases must have very similar densities in order to achievethe concentricity of the nucleus of the microsphere, since the relationof velocities between the gases moves according to the square root ofthe densities v1/v2=(ρ2/ρ1)^(½) n and both jets, the inside jet and theoutside jet, must assume the most symmetrical configuration possible,which does not occur if the fluids have different velocities (FIG. 2).Nonetheless, it has been experimentally demonstrated that, on account ofthe surface tension γ2 between the two fluids, the nucleus tends tomigrate toward the center of the microsphere, within prescribedparameters.

The distance between the plane of the internal tube 31 (the one thatwill normally project more) and the plane of the orifice may varybetween zero and three outside diameters of the external tube 32,depending on the surface tensions between the fluids and with theliquid, and on their viscosity values. Typically, the optimal distanceis found experimentally for each particular configuration and each setof liquids used.

The proposed dispersion system obviously requires fluids that are goingto be used in the resulting bubbles to have certain flow parameters.Accordingly, flows for this use must be:

Flows that are suitable so that the system falls within the parametricwindow of stability. Multiplexing (i.e. several sets of concentrictubes) may be used, if the flows required are greater than those of anindividual cell.

Flows that are suitable so that the mass relation of the fluids fallswithin the specifications of each application. Of course, a greater flowof liquid may be supplied externally by any means in specificapplications, since this does not interfere with the functioning of theatomizer.

If the flows are varied, the characteristic time of this variation mustbe less than the hydrodynamic residence times of liquid and gas in themicrojet, and less than the inverse of the firs; natural oscillationfrequency of the drop formed at the end of the injection needle.

Therefore, any means for continuous supply of gas (compressors, pressuredeposits, etc.) and of liquid (volumetric pumps, pressure bottles, etc.)may be used. If multiplexing is desired, the flow of gas must be ashomogeneous as possible between the various cells, which may requireimpulse through multiple capillary needles, porous media, or any othermedium capable of distributing a homogeneous flow among differentfeeding points.

Each dispersion device will consist of concentric tubes 31, 32 with adiameter ranging between 0.05 and 2 mm, preferably between 0.1 and 0.4mm, on which the drop from which the microjet emanates can be anchored,and a small orifice (between 0.001 and 2 mm in diameter, preferablybetween 0.1 and 0.25 mm), facing the drop and separated from the pointof feeding by a distance between 0.001 and 2 mm, preferably between 0.2and 0.5 mm. The orifice puts the liquid that surrounds the drop, athigher pressure, in touch with the area in which the dispersion is to beattained, at lower pressure.

Embodiment of FIG. 3

The embodiments of FIGS. 1 and 2 are similar in a number of ways. Bothhave a feeding piece which is preferably in the form of a feeding needlewith a circular exit opening. Further, both have an exit port in thepressure chamber which is positioned directly in front of the flow pathof fluid out of the feeding source. Precisely maintaining the alignmentof the flow path of the feeding source with the exit port of thepressure chamber can present an engineering challenge particularly whenthe device includes a number of feeding needles. The embodiment of FIG.3 is designed to simplify the manner in which components are aligned.The embodiment of FIG. 3 uses a planar feeding piece (which by virtue ofthe withdrawal effect produced by the pressure difference across a smallopening through which fluid is passed) to obtain multiple microjetswhich are expelled through multiple exit ports of a pressure chamberthereby obtaining multiple dispersionl streams. Although a single planarfeeding member as shown in FIG. 3 it, of course, is possible to producea device with a plurality of planar feeding members where each planarfeeding member feeds fluid to a linear array of outlet orifices in thesurrounding pressure chamber. In addition, the feeding member need notbe strictly planar, and may be a curved feeding device comprised of twosurfaces that maintain approximately the same spatial distance betweenthe two pieces of the feeding source. Such curved devices may have anylevel of curvature, e.g. circular, semicircular, elliptical,hemi-elliptical, etc.

The components of the embodiment of FIG. 3 are as follows:

41. Feeding piece.

42. End of the feeding piece used to insert the gas to be dispersed.

43. Pressure chamber.

44. Orifice used as liquid inlet.

45. End of the feeding needle used to evacuate the gas to be dispersed.

46. Orifices through which withdrawal takes place.

47. Dispersion bubbles.

48. First fluid containing material to be dispersed.

49. Second fluid for creation of microjet.

50. Wall of the propulsion chamber facing the edge of the feeding piece.

51. Channels for guidance of fluid through feeding piece.

d_(j)=diameter of the microjet formed; ρ_(A)=density of first fluid(48); ρ_(B)=density of second fluid (49); ν_(A)=velocity of the firstfluid (48); ν_(B)=velocity of the second fluid (49); e=axial length ofthe orifice through which withdrawal takes place; H=distance from thefeeding needle to the microjet outlet; P_(o)=pressure inside thechamber; Δp_(g)=change in pressure of the gas; P_(a)=atmosphericpressure; Q=volumetric flow rate

The proposed dispersion device consists of a feeding piece 41 whichcreates a planar feeding channel through which a where a first fluid 48flows. The flow is preferably directed through one or more channels ofuniform bores that are constructed on the planar surface of the feedingpiece 41. A pressure chamber 43 that holds the propelling flow of asecond liquid 49, houses the feeding piece 41 and is under a pressureabove maintained outside the chamber wall 50. One or more orifices,openings or slots (outlets) 46 made in the wall 52 of the propulsionchamber face the edge of the feeding piece. Preferably, each bore orchannel of the feeding piece 41 has its flow path substantially alignedwith an outlet 46.

When the second fluid 49 is a liquid and the first fluid 48 is a gas,the facts that the liquid is much more viscous and that the gas is muchless dense virtually equalize the fluid and gas velocities. The gasmicrothread formed is much shorter; however, because its rupture zone isalmost invariably located in a laminar flowing stream, dispersion in thesize of the microbubbles formed is almost always small. At a volumetricgas flow-rate Q_(g) and a liquid overpressure ΔP_(l), the diameter ofthe gas microjet is given by$d_{j} \cong {\left( \frac{8\rho_{l}}{\pi^{2}\Delta \quad P_{l}} \right)^{\frac{1}{4}}Q_{g}^{\frac{1}{2}}}$

The low liquid velocity and the absence of relative velocities betweenthe liquid and gas lead to the Rayleigh relation between the diametersof the microthread and those of the bubbles (i.e. d=1.89d_(j)).

Oxygenation of Water

More fish die from a lack of oxygen than any other cause. Fish exposedto low oxygen conditions become much more vulnerable to disease,parasites and infection, since low oxygen levels will (1) lower theoxidation/reduction potential (ORP) (2) favor growth of disease causingpathogens and (3) disrupt the function of many commercially availablebiofilters. Moreover, stress will reduce the fish activity level, growthrate, and may interfere with proper development. A continuous healthyminimum of oxygen is approximately a 6 parts per million (ppm)oxygen:water ratio, which is approximately 24 grams of dissolved oxygenper 1000 gallons of water. Fish consume on average 18 grams of oxygenper hour for every ten pounds of fish. Low level stress and poor feedingresponse can be seen at oxygen levels of 4-5 ppm. Acute stress, nofeeding and inactivity can be seen at oxygen levels of 2-4 ppm, andoxygen levels of approximately 1-2 ppm generally result in death. Thesenumbers are merely a guideline since a number of variable (e.g., watertemperature, water quality, condition of fish, level of other gasses,etc.) all may impact on actual oxygen needs.

Proper aeration depends primarily on two factors: the gentleness anddirection of water flow and the size and amount of the air bubbles. Withrespect to the latter, smaller air bubbles are preferable because they(1) increase the surface are between the air and the water, providing alarger area for oxygen diffusion and (2) smaller bubbles stay suspendedin water longer, providing a greater time period over which the oxygenmay diffuse into the water.

The technology of the invention provides a method for aerating water forthe proper growth and maintenance of fish. A device of the invention forsuch a use would provide an oxygenated gas, preferably air, as the firstfluid, and a liquid, preferably water, as the second fluid. The airprovided in a feeding source will be focused by the flow of thesurrounding water, creating a stable cusp at the interface of the twofluids. The particles containing the gas nucleus, and preferably airnucleus, are expelled into the liquid medium where aeration is desired.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, material, composition of matter, process, processstep or steps, to the objective, spirit and scope of the presentinvention. All such modifications are intended to be within the scope ofthe claims appended hereto.

What is claimed is:
 1. A monodispersion of bubbles in a food, comprisinga beverage and at least about one thousand bubbles, wherein the bubbleshave a diameter in a range of from about 0.1 micron to about 100 micronsand wherein the dispersion is characterized by substantially all thebubbles having approximately the same diameter with a deviation indiameter of about ±3%.
 2. The monodispersion of claim 1, wherein thebubbles are comprised of a gas chosen from air, oxygen, and carbondioxide.
 3. The monodispersion of claim 1, wherein the beverage isaqueous.
 4. The monodispersion of claim 2, wherein the gas is carbondioxide and the beverage is aqueous.
 5. The monodispersion of claim 1,wherein the bubbles are created by a flow stream of gas from a sourcethrough a liquid beverage in a pressure chamber, and wherein the liquidbeverage is forced out of an exit orifice of the pressure chamber whilesurrounding and focusing the flow stream of gas which exits the pressurechamber where the focused flow stream of gas breaks up to form thebubbles.
 6. A monodispersion as set forth in claim 1, formed by aprocess comprising: forcing a gas from a source opening into a firstliquid beverage in a manner so as to create a flow stream of the gasthrough the first liquid beverage, wherein the gas is comprised ofmolecules to be diffused into a second liquid beverage; moving the firstliquid beverage in a pressure chamber surrounding the source opening,out of an exit orifice in the pressure chamber wherein the flow streamof the gas flows out the exit orifice into the second liquid beveragewherein the flow stream breaks up forming bubbles of the gas in thesecond liquid beverage.
 7. The monodispersion of claim 6, wherein themolecules in the gas diffuse into the second liquid beverage.
 8. Themonodispersion of claim 6, wherein the bubbles are emitted at regularlyspaced intervals from the exit orifice of the pressure chamber.
 9. Themethod of claim 6, wherein the liquid beverage is aqueous.
 10. Themethod of claim 9, wherein the gas is carbon dioxide.
 11. Amonodispersion of bubbles in a beverage, comprising a beverage and atleast about one thousand bubbles, wherein the bubbles have a diameter ina range of from about 0.1 micron to about 100 microns and wherein thedispersion is characterized by substantially all the bubbles havingapproximately the same diameter with a deviation in diameter of about±3%, wherein said monodispersion is formed by a process comprising thefollowing steps: forcing a gas from a source opening into a first liquidbeverage in a manner so as to create a flow stream of the gas throughthe first liquid beverage, wherein the gas is comprised of molecules tobe diffused into a second liquid beverage; moving the first liquid in apressure chamber surrounding the source opening, out of an exit orificein the pressure chamber wherein the flow stream of the gas flows out theexit orifice into the second liquid beverage wherein the flow streambreaks up forming bubbles of the gas in the second liquid beverage. 12.The monodispersion of claim 11, wherein the bubbles are comprised of agas chosen from air, oxygen, and carbon dioxide.
 13. The monodispersionof claim 11, wherein the beverage is aqueous.
 14. The monodispersion ofclaim 12, wherein the gas is carbon dioxide and the first and secondliquid beverage are aqueous.
 15. The monodispersion of claim 11, whereinthe bubbles are emitted at regularly spaced intervals from the exitorifice of the pressure chamber.
 16. A beverage mixture comprisingdiffused components, wherein said components are diffused into thebeverage mixture by a process comprising the following steps: forcing agas from a source opening into a first liquid beverage in a manner so asto create a flow stream of the gas through the first liquid beverage,wherein the gas is comprised of molecules to be diffused into a secondliquid beverage; moving the first liquid beverage into a pressurechamber surrounding the source opening, out of an exit orifice in thepressure chamber wherein the flow stream of the gas flows out the exitorifice into the second liquid beverage wherein the flow stream breaksup forming bubbles of the gas in the second liquid beverage, and whereinsaid second liquid beverage and said bubbles of gas form a mixture ofdiffused components and further wherein the bubbles are emitted atregularly spaced intervals from the exit orifice of the pressure chamberand substantially all the bubbles have approximately the same diameterwith a deviation in diameter of about ±3%.
 17. The mixture of claim 16,wherein the bubbles are comprised of a gas chosen from air, oxygen, andcarbon dioxide.
 18. The mixture of claim 16, wherein the first liquidbeverage is aqueous.
 19. The mixture of claim 16, wherein the gas iscarbon dioxide and the first liquid beverage is aqueous.
 20. The mixtureof claim 16, wherein the bubbles are emitted at regularly spacedintervals from the exit orifice of the pressure chamber.
 21. A liquidbeverage comprising a monodispersion of bubbles, wherein saidmonodispersion comprises at least about one thousand bubbles, whereinthe bubbles have a diameter in a range of from about 0.1 micron to about100 microns and wherein substantially all the bubbles have approximatelythe same diameter with a deviation in diameter wherein the bubbles arediffused into a liquid by a process comprising the following steps:forcing a gas from a source opening into a first liquid beverage in amanner so as to create a flow stream of the gas through the firstliquid, wherein the gas is comprised of molecules to be diffused into asecond liquid beverage; moving the first liquid in a pressure chambersurrounding the source opening, out of an exit orifice in the pressurechamber wherein the flow stream of the gas flows out the exit orificeinto the second liquid beverage wherein the flow stream breaks upforming a monodispersion of bubbles of the gas in the second liquidbeverage.
 22. The monodispersion of claim 21, wherein the bubbles arecomprised of a gas chosen from air, oxygen, and carbon dioxide.
 23. Themonodispersion of claim 21, wherein the first liquid beverage isaqueous.
 24. The monodispersion of claim 22, wherein the gas is carbondioxide and the first and second liquid beverage are aqueous.
 25. Themonodispersion of claim 21, wherein the bubbles are emitted at regularlyspaced intervals from the exit orifice of the pressure chamber.